The goal of this project is the delineation of the antigens recognized by naturally occurring anti-platelet antibodies. These will be studied using several different approaches. Initial studies have been performed demonstrating the utility of radioimmunoassays for detection of platelet-associated IgG and C3. Subsequent studies have demonstrated the presence of heparin dependent IgG antibodies in some patients with heparin-associated thrombocytopenia and the involvement of complement in mediating platelet injury. The interaction of 125I-labeled solubilized platelets with platelet antibodies will be studied. The antigens will be isolated by indirect immunoprecipitation using purified human IgG anti-platelet antibodies and rabbit antihuman IgG. In addition, platelet antibodies will be coupled to sepharose and the platelet antigens isolated by affinity chromatography. Following either extraction procedure, the apparent molecular weight of the antigen will be identified on SDS-polyacrilamide gel electrophoresis. Clinically diverse patients with autoreactive platelet antibodies will be similarly studied. In addition, the localization of antigens on the platelet surface will be studied with electron microscopy. Fab' antibodies from patients with isoimmune or autoimmune platelet disorders will be isolated. The platelet antibodies will then be coupled to hemocyanin or ferritin interacted with platelets and both scanning EM and transmission EM studies performed.